1. Field of Endeavor
The present invention relates to the electrochemical formation of hydroxide. More particularly the present invention relates to electrochemical formation of hydroxide from mineral carbonate for enhancing carbon dioxide and acid gas uptake by a solution. The invention can also generate hydrogen gas and oxidative gases such as oxygen or chlorine.
2. State of Technology
Due to the climate and environmental effects of excess carbon dioxide (CO2) in the atmosphere, a variety of methods exist or have been proposed for pre- or post-emission capture and sequestration of CO2. For example, it is well known that CO2 will react with hydroxides in solution such that the CO2 contained in a gas mixture in contact with such a solution will be reduced via absorption and reaction within the solution, and such reactions have industrial applications. More recently the use of solutions containing calcium hydroxide (Ca(OH)2) or sodium hydroxide (NaOH) have been proposed for large-scale chemical absorption of air CO2 using various means of active or passive contacting of air or a gas mixture and the solution. For example, Kheshgi (Kheshgi, H. S. Sequestering atmospheric carbon dioxide by increasing ocean alkalinity. Energy 1995, 20, 915-922) suggested placing calcium oxide (CaO) or Ca(OH)2 in the ocean to effect passive uptake of CO2 from the atmosphere, largely forming calcium bicarbonate (Ca(HCO3)2) in solution as the CO2 storage product. Other schemes employ engineered structures for the contacting of air with NaOH, forming sodium carbonate (Na2CO3) in solution (e.g., Zeman, F. Energy and material balance of CO2 capture from ambient air Environ. Sci. Technol. 2007, 41, 7558-7563; US Patent Application 2006/0051274 A1; US Patent Application 2006/0093540 A1). By subsequently reacting this solution with Ca(OH)2, calcium carbonate (CaCO3) is formed and NaOH is regenerated. The CaCO3 is then calcined at high temperature to form concentrated CO2 as the final storage product while also forming CaO. The latter is then hydrated to regenerate Ca(OH)2. In this way alkaline hydroxide solutions are recycled and conserved, as opposed to the once-through production and release of alkalinity in the concept proposed by Kheshgi. However, in both cases significant quantities of thermal energy are required to either produce or regenerate the hydroxide solutions, especially the calcination of CaCO3. This contributes significantly to the cost of either process, plus additional CO2 is produced if the source of the thermal energy is derived from the combustion of fossil fuels.
Another source of hydroxide is electrochemical salt splitting wherein a dissolved salt is split into acid and hydroxide components in the presence of a charged anode and cathode, respectively. For example a solution containing dissolved sodium chloride (NaCl) can be electrolyzed to form hydrochloric acid (HCl), hypochlorite (ClO−), chlorate (ClO3−), and/or chlorine gas (Cl2) at the anode and NaOH at the cathode. The hydroxide solution can then be removed for subsequent use. Obviously, such electrochemically-produced hydroxide solutions could be used for CO2 and other acid gas mitigation (e.g., U.S. Pat. Nos. 3,344,050, 3,692,649, 3,801,698; US Patent Application 2006/0185985). However, producing hydroxide in quantities sufficient for large scale CO2 removal could, in the case of a metal chloride-containing electrolyte, mean massive co-production of one or more chlorine-containing compounds. If these were not consumed in appropriate ways they would pose a significant environmental impact.
U.S. Pat. No. 4,337,126 discloses electrolysis of carbonates to produce hydroxides. The process is directed to electrolytic production of hydroxides of alkali metal from alkali metal carbonates contained in waste streams and naturally occurring carbonate and/or bicarbonate deposits or ores. Alkali metal carbonates are produced as by-products in a variety of processes which rely on other alkali metal salts or alkali metal hydroxides as reactants or as treating agents. However, the alkali metal carbonates used, in particular potassium carbonate, are introduced into the process in dissolved form and therefore preclude the use of more abundant but insoluble alkali metal carbonates such as calcium carbonate or magnesium carbonate. CO2 production rather than CO2 mitigation is effected by the invention.
U.S. Pat. No. 5,246,551 discloses electrochemical methods for production of alkali metal hydroxides without the co-production of chlorine. Alkali metal hydroxides are manufactured in the United States at the rate of approximately 36,500 tons/day, almost entirely by the electrolysis of aqueous brine solutions, but resulting in the co-production of chlorine. Aqueous solutions of alkali metal carbonates and bicarbonates are used in the invention, in particular sodium carbonates and bicarbonates, requiring that the alkali metal carbonates and bicarbonates be in dissolved form prior to introduction to the system. This precludes the use of water insoluble alkali metal carbonates such as calcium or magnesium carbonate that can be much more abundant and less expensive than water soluble forms for large scale applications. CO2 is also produced rather than mitigated by this process.
United States Published Patent Application No. 2006/0185985 discloses removing carbon dioxide and other pollutants from a gas stream by contacting with an electrochemically generated hydroxide solution to form metal carbonate and/or bicarbonate. However, in this case the metal source for the produced hydroxide solution is derived from a soluble metal chloride salt, in particular sodium chloride, requiring the formation of chlorine-containing compounds.
The referenced shortcomings of the preceding methods of hydroxide production and CO2 mitigation are not intended to be exhaustive, but rather are among those that impair or limit their application. A number of these shortcomings are overcome by the techniques described and claimed in this disclosure.